What is stainless steel

This article covers the following topics;

  • What is stainless steel?
  • Types of stainless steel.
  • Is stainless steel magnetic?
  • Stainless steel welding and weldability.
  • How to select the welding rod or filler wire?

we will start this article with the first topic/question i.e what is stainless steel?

WHAT IS STAINLESS STEEL

stainless steels (SS) are essentially iron base alloy steels containing at least 10.5 % Chromium. Other important alloying elements that may be present in stainless steel are carbon, Nickel, Manganese, etc. “SS’ is an abbreviation used for stainless steel in the industries.

Due to the presence of chromium, a very thin chromium-rich oxide layer is formed on the outer surface of stainless steel. This chromium-rich oxide layer has two unique features;

  1. Passive layer: Due to passive (inactive) in nature, this layer does not reacts with the environment (especially oxygen – the major cause of rusting) and prevents oxidation hence the stainless steel remains safe and free from rust.
  2. Self-repairable: Whenever the chromium-rich oxide layer is damaged, a new layer is formed quickly. Hence, the stainless steel will remain rust-free. However, the rate at which the chromium oxide passive film is developed depends on its chromium content.

Polished stainless steel remains bright under most environmental conditions.

 

TYPES OF STAINLESS STEELS

stainless steel

There are five main types (or grades) of stainless steel, these are the following;

  1. Austenitic stainless steel – FCC (face-centered cubic) crystal structure
  2. Ferritic stainless steel – BCC (body-centered cubic) crystal structure
  3. Martensitic stainless steel – BCT (body-centered tetragonal) crystal structure
  4. Duplex stainless steel – FCC + BCC i.e. mixture of Austenite and Ferrite
  5. Precipitation-hardening (PH) stainless steel

Out of these five types, the first four i.e. Austenitic, Ferritic, Martensitic, and duplex are categorized according to their crystal structure and if they are additionally strengthened by precipitation hardening process then the product obtained is known as Precipitation-Hardening (PH) stainless steel.

A very important question, which comes to our mind, is;

Is stainless steel magnetic? Or which type of stainless steels are magnetic?

In general, the Austenite stainless steels are non-magnetic, a term paramagnetic is also used for non-magnetic elements. Hence, we can say that Austenitic stainless steels are paramagnetic. This concept will be discussed later on in this article

In addition to the above grades, some advanced grades (or specialty grades) of stainless steels are also being used in the industries, these are;

  • Superaustenitic Stainless steel
  • Superferritic stainless steel
  • Supermartensitic stainless steel
  • Superduplex stainless steel

The austenitic stainless steel can be further divided into two types;

  1. Austenitic stainless steel containing Chromium and Nickel as the main alloying elements (In addition to Iron) – These are identified as AISI 300 Series types.
  2. Austenitic stainless steel containing Chromium, Nickel, and Manganese as the main alloying elements (In addition to Iron) – These are identified as AISI 200 Series types.

Ferritic stainless steels contain chromium as the major alloying element and are identified as AISI 400 series types.

Martensitic stainless steels also contain chromium as the main alloying element (In addition to Iron and Carbon) and identified are AISI 400 series types.

Table 1, gives a general summary of types of Stainless steel and their corresponding identification No. (As per the AISI Classification system), detrimental alloying elements, Main types/grades, and P Numbers (As per ASME Sec IX).

Table – 1

Type of Stainless Steel

AISI Classification systemMajor Alloying ElementsMain types/grades (AISI)P number

(ASME Sec IX)

Austenitic stainless steel

 

2XX

Chromium + Nickel + Manganese

201

202

3XXChromium + Nickel301

302, 302 B

303. 303Se

304, 304H, 304L, 304LN, 304N

305

308

309, 309S

310, 310S

316, 316H, 316L

317, 317L

321

330

334

347

348

384

 

8

Ferritic stainless steel

4XXChromium 

405

409 – 10/20/30

429

430, 430Ti

434

436

439

442

444

446

 

7

Martensitic stainless steel403

410

414

416

420

422

431

440A, 440B, 440C

6

 

MARTENSITIC STAINLESS STEEL

Martensitic stainless steels were the first stainless steels to be produced in the world. As mentioned above they are essentially iron-chromium-carbon alloys with a nominal of 11.5% to 18% chromium.

They are hardenable by appropriate heat treatments and can also be hardened by cold working.

Martensitic stainless steel can be transformed into austenite when heated beyond 1010°C (1850°F). However, rapid cooling from this temperature will again result in a martensitic microstructure.

These steels are popular for

  • Relatively low cost.
  • Moderate corrosion resistance,
  • Oxidation resistance,
  • Lacks toughness and require tempering for adequate toughness
  • Ability to develop a wide range of mechanical properties

Martensitic stainless steels are used to fabricate a variety of products, for example, Low and medium carbon martensitic stainless steels are typically used in jet engines, steam turbines, and gas turbines. High carbon martensitic stainless steels are used for gears, shafts, cams, ball bearings, and valves, etc.

Weldability of Martensitic stainless steel:

Stainless steel welding

Martensitic stainless steels often produce hardened HAZs, and as the hardness of HAZ increases, it’s toughness decreases, and susceptibility to Hydrogen induced cracking increases.

As a general practice, post weld heat treatment (PWHT) is given to martensitic stainless steel welded joints, to improve the weld properties.

It’s weldability, in general, increases when an austenitic type filler metal or welding rod is used.

Since Martensitic stainless steels are subject to hydrogen-induced cracking hence proper precautions must be taken in the selection of welding process, handling, and storage of the filler metal and cleanliness to avoid hydrogen from entering into the weld metal.

Following Welding processes can be employed to weld Martensitic stainless steel

1. Arc Welding
  • Shielded Metal Arc Welding (SMAW) or stick welding
  • Gas Metal Arc welding (GMAW) or MIG Welding
  • Gas Tungsten Arc Welding (GTAW) or TIG welding
  • Flux Cored Arc Welding (FCAW)
  • Plasma Arc Welding (PAW)
  • Submerged Arc welding (SAW)
2. Resistance Welding
  • Resistance Spot Welding
  • Resistance Flash Welding
3. Electron beam welding
4. Laser beam welding
5. Friction welding
6. High frequency welding

Filler metal/Welding Rod for martensitic type stainless steels:

Three most commonly used filler metal grades for martensite stainless steel are;

  • E410/ER410
  • E410 NiMo /ER410 NiMo
  • E420/ER420

Filler metal type 410 (E410/ER410) can be used to weld following martensitic stainless Types;

  • 403
  • 410
  • 414
  • 420

Type 410 NiMo filler metal is used to weld Type CA-6NM castings (cast martensite stainless steel)

Filler metal ER420 is used to weld type 420 stainless steel when the main goal is matching the carbon content of base metal with filler metal. This filler metal may also be used for surfacing of carbon steels to provide good corrosion and wear resistance.

However, martensitic stainless steel welding lacks good toughness properties (except for ER410NiMo), hence PWHT is carried to achieve good toughness (If required).

To achieve good weld metal toughness property, austenitic stainless steel filler metal type 308 (E308/ER308) & 309 (E309/ER309) can also be used to weld martensitic stainless steels to martensitic stainless steels or any other types of stainless steels.

For Types 416 and 416Se steels that are free-machining grades, E312-15 austenitic stainless steel filler metal may be used for welding.

Preheat and PWHT requirements:

Preheat and post weld heat treatment (PWHT) requirements for martensitic stainless steel are given in Table – 2.

Table – 2

Carbon content (%)

Preheat temperature (minimum)Requirements for PWHT

°C

°F

<0.05

121250Optional

0.05–0.15

204

400Recommended
>0.15316600

Mandatory

FERRITICTIC STAINLESS STEEL

Ferritic stainless steels are essentially iron-chromium-carbon alloys with a nominal of 11% to 30% chromium along with other ferrite stabilizers, such as molybdenum, aluminum, niobium, or titanium.

They possess a body-centered cubic (BCC) crystal structure. These steels exhibit good ductility and have good resistance to stress corrosion cracking, pitting, and crevice corrosion.

Ferritic stainless steels with Low chromium (Approx 11%) such as Type 409, are commonly used in automotive exhaust systems. Ferritic stainless steel alloys having an intermediary level of chromium content (16% to 18%) are often used in food handling and automotive trim applications. High chromium content ferritic stainless steel with additions of molybdenum (often referred to as superferritic stainless steels) are commonly used in applications that require high levels of oxidation and corrosion resistance such as heat exchangers and piping systems for seawater.

Types 430, 442, and 446 are referred to as the first-generation ferritic stainless steels. They contain mainly chromium as a ferrite stabilizer along with relatively high carbon content.

They often require PWHT otherwise intergranular corrosion may occur. They also exhibit low toughness.

Whereas, Types 405 and 409 are referred to as the second-generation ferritic stainless steels. They have lower chromium and carbon content but contain ferrite formers.

These steels are also referred to as pseudoferritic because they require other ferrite formers in addition to chromium.

They are comparatively less costly, possess good fabrication characteristics, and have useful corrosion resistance than the first-generation ferritic stainless steels but they often possess low toughness.

Weldability of ferritic stainless steel:

Generally, fewer precautions are required during welding because they cannot be hardened by quenching. Hence, the chances of martensite formation are less during the cooling of weld metal. However, Types 430, 434, 442, and 446 are exceptional cases due to the presence of both high chromium and high carbon content. The risk of hydrogen-induced cracking during cooling is more in these alloys especially when welding is carried out under high restraint conditions such as heavy weldments or surfacing welds on carbon steel. To minimize residual stresses that contribute to weld, preheating of 150°C (300°F) or higher can be used.

Chances of Hydrogen embrittlement increases in ferrites stainless steel when martensite is present along ferrite grain boundaries in the weld metal or HAZ. However, Ferritic stainless steels are less susceptible to hydrogen embrittlement if compared to martensitic stainless steel.

The risk of solidification cracking in ferritic stainless steels is comparatively very less because the primary solidification phase is ferrite. However, Alloys with additional alloying elements like titanium and niobium or high impurity levels are more susceptible to solidification cracking

Ferritic Stainless Filler Metals:

To weld ferritic stainless steels with ferritic stainless steels or to any dissimilar steel, Filler metal/welding electrode of following types can be selected;

  1. Filler metals with compositions approximately matching to those of the base metals
  2. Austenitic stainless filler metals (Types 309 and 312)
  3. Nickel-alloy filler metals (ERNiCr-3, ENiCrFe-2, or ENiCrFe-3)

Filler metals/welding rods matching Types 409 and 430 stainless steels (base metal) are widely available. However, whenever ferritic stainless steels are used as filler metal, the resulting welds lack in toughness properties in both the weld metal and the HAZ

Filler metals made up of Austenitic stainless steel such as Types 309 and 312 or nickel-alloy filler metals such as ERNiCr-3, ENiCrFe-2, or ENiCrFe-3 often are selected for joining ferritic stainless steels to ferritic stainless steels or any dissimilar metals.

Type 444 stainless steel can be welded to matching steel with Type 316L weld metal and Type 430 steel can be welded with E308 and E308L.

when welding ferritic stainless steels to ferritic stainless steels or mild or low alloy steels, nickel alloys, and copper-nickel alloys, Nickel-alloy filler metal, such as ERNiCr-3, ENiCrFe-2, or ENiCrFe-3, can be used which produce sound weld joints.

However, Austenitic stainless steels are generally less resistant to stress corrosion cracking (SCC) than ferritic stainless steel alloys. Hence, proper consideration must be given before choosing filler metals.

To weld Low-chromium ferritic stainless steels, such as 405 and 409 with mild steel, carbon steel filler metals can be used with proper care to avoid excessive dilution.

Preheat and PWHT requirements:

The preheating requirements are determined largely by job thickness, chemical composition, desired mechanical properties, and restraint conditions.

Ferritic stainless steels with low chromium or high-carbon content can be preheated within the range of 150°C to 230°C (300°F to 450°F).

PWHT for first-generation ferritic stainless steels (Types 430, 442, and 446) can be conducted at temperatures ranging from 700°C to 840°C (1300°F to 1550°F). These temperature ranges help prevent further grain coarsening.

Whereas, PWHT for Second-generation ferritic stainless steels (Types 405 and 409) can be conducted at higher temperatures up to at least 1040°C (1900°F)

 

AUSTENITIC STAINLESS STEEL

Austenitic stainless steels are the most widely used stainless steels in the world. They have a face-centered cubic (FCC) crystal structure and are nonmagnetic (also known as paramagnetic) in the annealed condition. However, the magnetic properties of austenitic stainless steel can be increased by cold working.

The chromium content is generally above 16% in austenitic stainless steels and total chromium, nickel, manganese, and silicon content over 25% by weight, They are popular among industries for;

  • Good ductility
  • Excellent strength
  • Good corrosion resistance
  • High toughness
  • Excellent cryogenic properties
  • Excellent strength and oxidation resistance at high temperatures.

Austenitic stainless steel based mainly on the Iron-Chromium-Manganese-Nitrogen is identified by a three-digit number system starting with 2, such as 201 and 202. Whereas, alloys based on the Iron –Chromium-Nickel-Carbon are also identified by a three-digit number system but starting with 3 for example, 304 and 309, etc.

Due to face-centered-cubic (FCC) crystal structure, austenitic stainless steels have better toughness and ductility than carbon steels and alloy steels. The notch toughness at cryogenic temperatures is also excellent.

Type 316H stainless steels possess the best stress-rupture behavior of the Series 300 austenitic stainless steel.

Weldability of austenitic stainless steel:

Austenitic stainless steels possess higher thermal expansion than the ferritic or martensitic stainless steels.

Distortion or warping occurs during the welding of austenitic stainless steel due to it’s high coefficient of thermal expansion and low thermal conductivity.

Austenitic stainless steel is susceptible to solidification and liquation cracking. Hence, proper care to be given while selecting filler material and welding process.

Submerged arc welding (SAW) is not preferred when fully austenitic stainless steel or low ferrite content weld deposit is required.

Table – 3, gives a guide for the selection of proper filler wire or welding rods as per the base material (for austenitic stainless steel).

Table – 3

Type of austenitic stainless steel

Filler Metal/Welding rods
SMAW

(Covered Electrodes)

GMAW, GTAW, PAW, SAW

(Bare welding rods)

FCAW

(Tubular flux-cored welding rods)

201, 202

E308, ER209, E219ER308, ER209, ER219E308TX-X

301, 302, 304, 305

E308ER308

E308TX-X

304L

E308L, E347ER308L, ER347

E308LTX-X, E347TX-X

309

E309ER309

E309TX-X

309SE309L, E309CbER309L

E309LTX-X, E309CbLTX-X

310, 314

E310ER310

E310TX-X

310S

E310, E310CbER310

E310TX-X

316

E316ER316

E316TX-X

316L

E316LER316L

E316LTX-X

316H

E16-8-2, E316HER16-8-2, ER316H

E316TX-X

317

E317ER317

E317LTX-X

317L

E317LER317L

E317LTX-X

321

E308L, E347ER321

E308LTX-X, E347TX-X

330

E330

ER330

347, 348

E308L, E347ER347

E308LTX-X, E347TX-X

 

N – Addition of Nitrogen

H – High Carbon content

L – Low Carbon content

Sandeep Anand

I am a Mechanical Engineer with more than ten years of work experience in the field of welding and NDT.

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